In LTE (Long Term Evolution) specified in 3rd Generation Partnership Project (3GPP), VoLTE (Voice over LTE) is used to connect a calling subscriber to a voice call partner through an IP multimedia subsystem (IP (Internet Protocol) Multimedia Subsystem: IMS). VoLTE provides voice communication as data communication (packet communication) in the LTE scheme. The VoLTE system includes, as its system configuration, a VoLTE-compatible terminal (User Equipment (UE)), LTE/EPC (Evolved Packet Core), and IMS. A method for connecting an emergency call via IMS to a PSAP (Public Safety Answering Point: Emergency Notification Acceptance Center) such as Police, Fire Department and Emergency etc. is specified in 4.1 Architectural Principles of Non-Patent Literature 1 and the like. IMS emergency call means an emergency call provided by IMS (for example, 110 in Japan). Emergency calls fall into a category of local service as a definition of telecom. In other words, dial numbers such as 110 and 119 are effective only in a relevant area (=in Japan). Emergency numbers in Europe (police, emergency, firefighting) are 122 (police in the UK is 110), which are effective only in Europe.
When a terminal (UE) transmits an IMS emergency call, the terminal (UE) sets up a connection (Emergency PDN connection) to a PDN (Packet Data Network) designated for emergency call. If the terminal is not registered in the IMS, the terminal carries out IMS Emergency Registration processing and then transmits a SIP (Session Initiation Protocol) INVITE request indicating an emergency call to a P-CSCF (Proxy Call Session Control Function) within the IMS. After the IMS receives the INVITE request, the IMS selects a connection destination of the emergency call, based on location information of the terminal. For example, after the P-CSCF receives the INVITE request from the terminal, the P-CSCF forwards the INVITE request to an E-CSCF (Emergency Call Session Control Function), which is a SIP server controlling the emergency call. The E-CSCF acquires, for example, cell identification information of a serving cell of the terminal as location information of the terminal via a LRF (Location Retrieval Function). The E-CSCF selects a route to a PSAP (Public Safety Answering Point) to be connected and then transmits the INVITE request. The LRF has a function of searching for location information of a terminal which has initiated an IMS emergency session and route information to the PSAP. It is noted that an emergency service URN (Emergency Service URN (uniform resource name)) (for example, urn: service: sos.police) is included in a “Request-URI (Uniform Resource Identifier)” of the INVITE request. Based on the emergency service URN, the E-CSCF sends a connection request for the emergency call to a police officer PSAP.
[Non-Patent Literature 1]
    3GPP TS 23.167V12.1.0 (2015-03) 4.1, 6.1, 7.1.2, 7.3[Non-Patent Literature 2]    3GPP TS 24.229V13.1.0 (2015-03) 5.1, 5.2, 7.2A.4[Non-Patent Literature 3]    3GPP TSG SA WG2 Meeting #109, S2-152061 (2015-05)